1. Field of the Invention
The present invention relates to an optical disc apparatus, and especially relates to stabilizing focusing on of a laser beam.
2. Description of the Related Art
Recently, an optical pickup for optical disc apparatus is proposed, in which a lens holder for holding an objective lens is slidably engaged with a guide shaft in parallel with an optical axis of the objective lens for stabilizing a posture of the objective lens of the optical pickup. In such an axial sliding type optical pickup, the lens holder is moved up and down along the guide shaft in parallel with an optical axis of the objective lens for executing the focusing servo control. The lens holder is reciprocally swung around the guide shaft for executing the tracking servo control.
For executing a focusing on operation in the optical disc apparatus having the axial sliding type optical pickup, a focusing servo mechanism is driven while the lens holder is swung out to an end in a predetermined direction around the guide shaft and the tracking servo mechanism is out of driving. There, however, is a problem that the motion of the lens holder is not smooth, since a bearing of the lens holder is forcibly slid along the guide shaft while the bearing is constantly pressed to the guide shaft in a predetermined direction. Specifically, the lens holder is hooked with the guide shaft due to adhesion of foreign substance such as dust between the bearing and the guide shaft or due to friction between surfaces of the bearing and the guide shaft, even when the focusing mechanism is driven so as to move the lens holder. Consequently, the lens holder cannot move immediately. When a driving force of the focusing mechanism is gradually increased and overcomes a load due to hooking of the lens holder with the guide shaft, the lens holder suddenly moves. Microscopically, the lens holder is moved up and down along the guide shaft with repeating the stop and the sudden movement due to the hooking and the release of the hooking.
For focusing the laser beam on a data recording face of the optical disc in the focusing on operation, it is necessary that the motion of the lens holder is smooth and the moving velocity of the lens holder is stable. On the contrary, since the lens holder repeats the sudden movement and the stop, the variation of the moving velocity of the lens holder becomes much larger. Consequently, it is highly possible that the laser beam is out of focus on the data recording face of the optical disc.